1. Field of the Invention
The present invention relates to a zoom lens system and a camera including the same.
2. Description of the Related Art
In recent years, a camera such as a video camera, a digital still camera, a broadcasting camera, and a monitoring camera which use a solid-state image pickup element, or a camera using a silver-halide film is improved in function, and the entire camera is reduced in size.
A zoom lens system which has a short entire lens length, is compact, has high zoom ratio, and has high resolution is demanded for an imaging optical system used for the camera.
As one of the zoom lens system that can meet the requirements, there is known a positive-lead zoom lens system in which a lens unit having a positive refractive power, a lens unit having a negative refractive power, and subsequently a rear unit including at least one lens unit are disposed in the stated order from an object side to an image side (Japanese Patent Application Laid-Open No. 2007-212537, Japanese Patent Application Laid-Open No. 2005-338740 (corresponding to U.S. Pat. No. 7,283,310), Japanese Patent Application Laid-Open No. 2007-171371 (corresponding to U.S. Pat. No. 7,319,562), Japanese Patent Application Laid-Open No. H06-160712, and Japanese Patent Application Laid-Open No. H06-337375 (corresponding to U.S. Pat. No. 5,642,224)).
In addition, a translucent ceramic material has been developed recently, and there is known a shooting optical system using the translucent ceramic material as an optical material. The translucent ceramic material has a refractive index larger than that of optical glass and is superior in hardness and strength. There is known a camera that utilizes this characteristic so as to achieve a thin profile of the entire lens system (Japanese Patent Application Laid-Open No. 2006-084887 (corresponding to U.S. Pat. No. 7,407,334)).
In general, a zoom lens system can be made smaller in size by enhancing refractive powers of lens units constituting the zoom lens system while reducing the number of lens elements.
However, the zoom lens system having such the structure may increase a lens thickness to cause insufficient effects of shortening the lens system and difficulty in compensating for various aberrations.
Therefore, in order to achieve a high zoom ratio and a small size of the entire lens system, it is necessary to set appropriately a zoom type, refractive powers of lens units, a lens structure of each lens unit, and the like.
In addition, optical glass usually has characteristics that when a refractive index is increased, an Abbe number is decreased while dispersion is increased.
In contrast, there is known a ceramic material having the same Abbe number as that of optical glass and a higher refractive index than that of optical glass.
If the ceramic material having such characteristics is used as an optical material, it is advantageous for compensating for aberration and achieving a small size of the entire optical system.
However, if the lens element made of a ceramic material is simply used for a zoom lens system, it is difficult to achieve a small size of the entire system and to obtain high optical performance over the entire zoom range.
In particular, as to the positive-lead zoom lens system described above, it is important to set appropriately a lens structure of a first lens unit for achieving a small effective diameter of the first lens unit, a small size of the entire system, and a high zoom ratio. For instance, it is important to select an appropriate material for the lens element of the first lens unit and to constitute the first lens unit with a minimum number of lens elements.
In the positive-lead zoom lens system, if the lens structure of the first lens unit is inappropriate, it is very difficult to achieve a small size of the entire system and to obtain high optical performance over the entire zoom range.